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Community and Q&A

Cape Cod–Building a maintainable insuated roof (on the cheap!)

Mark Waldron | Posted in Energy Efficiency and Durability on

We are purchasing a Cape Cod style home near Dayton, OH (climate 5A, but the southern edge). I’ve read many of the helpful articles here (thanks, Martin, for the notes about Capes here: https://www.greenbuildingadvisor.com/articles/dept/musings/insulating-cape-cod-house . I can’t say I wasn’t warned!)

Other articles I found very useful:
https://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-attic-venting

https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
This situation:
Money s a factor. E.g. I know I can’t afford to fill the rafter bays with professionally-applied spray foam.
HVAC equipment and ducting is in the area behind the knee wall.
Rafters are 2×6, decking is 1x7s on a 45 deg angle, and ceiling joists are 2x8s.
Present ceiling/roof insulation is an “all of the above” approach–fiberglass batts stapled to the rafters and up against the roof deck, more batts on top of the ceiling, more batts in the knee wall. Many of the batts are falling down, there has been water damage to the roof deck and ceilings, etc.
So, I have been struggling with the normal “big question”: Insulate under the roof deck OR
insulate the back of the knee wall and the top of the ceiling. Despite all the good advice in all the articles, I was initially leaning toward doing the ceiling and kneewall: blown cellulose is cheap, fast, and effective, also I really like the ability to see the back of the roof decking. All roofs eventually leak, and being able to see where this is happening is a giant plus in my book.
After thinking harder, I’m coming around to the idea of a vented “cathedral ceiling” under the roof deck. Top down: shingles and vapor permeable underlayment, 1×7 decking, 1.5″ ventilation channel, 4″ polyiso (FG facers, so approx perm rating of 1), then 1.25″ faced polyiso rated for exposure (Johns Manville CIMax–similar to Dow Thermax I think) screwed to the bottom of the rafters. This should get me to R31 between the rafters, with an R7.5 thermal break on their underside. This doesn’t meet code R-value, but it’s darn close from a practical standpoint and I’m hopeful the ventilation will provide some insurance against ice dams. Behind the sloped portion of the 2nd floor ceiling I’ll have the same cross-section, except I won’t need the pricey exposure-rated polyiso inside, it will just be 1″ of regular faced polyiso with gypsum on the interior. (I may sound like an advertisement for polyiso, but I’ve only got a few inches available here and it seems like the best choice.)
I decided to go with insulating the roofline rather than ceiling and kneewalls because:
1) My roof is very simple. We’re getting rid of the skylights, so it will just be two flues and some plumbing stacks penetrating the roof
2) The HVAC equipment and ducts are up here.
3) Tons of ceiling penetrations and imperfections (dropped soffiits, open-topped “wet walls” with
plumbing etc.)
4) Even if I’d gone with the knee wall insulation, I’d still have to put a cathedral ceiling behind the “gambrelled” portion of the second-floor ceiling.
5) All-in materials cost will come to about $1.80 PSF (less if I can find some reclaimed polyiso locally). Doing the ceiling-and-kneewall approach would have been about half the materials cost, but probably take longer (insulating HVAC, making the ceiling more airtight, etc). Despite the deeper wood available on the ceiling joists, the overall R-value wouldn’t have been much different unless I took steps to bring the kneewalls up to approx R30, and that would add more cost.

Finally, my questions–
1) Opinions on whether the “exposure rated” polyiso is needed in this attic? I know that it is okay to have exposed “regular” foam in this unoccupied space–but this house is for a relative, and I want to take prudent safety steps beyond what the code says if it makes sense. Are there better approaches to reduce fire threat?
2) From a practical perspective–how do people fill rafter/stud bays with rigid foam insulation slabs? My plan is to impale a couple of small scraps of 1.5″ thick EPS on the numerous protruding shingle nails to serve as my ventilation gap spacers, fit the 4″ thick polyiso slab into the bay, then use the foam gun to fill the gaps. But is it best to cut the foam 1″ narrow so I can get the gun tip up to the deck on each side? Do I need to fill the entire 4″ side of the foam billet with gun foam, or is nearly as good to just foam the top and bottom of each slab edge?
3) Should I truly “condition” the attic area (add supply and return ducting, etc)? Some articles have mentioned that it should incorporated into HVAC scheme to avoid moisture issues, and I don’t understand this. If I’ve got good air sealing to the outside, then the main air exchange will be with the interior living spaces of the house. In the summer, the attic (filled with that air from the living spaces) will be warmer, and thus of lower relative humidity. In the winter, I suppose the same reasoning (attic cooler than the rest of the house, same amount of moisture) would make the attic areas have higher RH than the living spaces, but our wintertime indoor RH is usually pretty low. Anyway, it’s no trouble to add some HVAC registers to these small areas behind the kneewall, but I would be interested to hear comments and reasoning.
4) Detecting and locating roof leaks. One advantage of a conventional vented attic: when the roof leaks, it drips into the fiberglass, wets the ceiling, and this provides a visible (unwelcome) indication of a big problem. With the layout I’ve described above I feel I may have a “nice on paper, but you’ll be sorry in 10 years” setup. Is there any way to retain the “early warning” of a roof leak and the ability to locate it? As crazy as it sounds, I’ve even thought of an electronic water sensor at the bottom of each roof deck (just one electronic alarm for each deck). Near the bottom of each rafter bay ventilation channel, include a bit of absorbent fabric. Put the parallel sensor wires in each one. In theory, any significant roof leak should wind up in the ventilation gap, and should run down to the bottom. The alarm goes off and you know there’s liquid water in that roof. A little crawling around and jamming long ohmeter probes through the foam should tell which rafter bay is leaking. Then work your way up that bay to find where the wetness is (staple individual fabric sheets to the top of each billet to make electronic location of water in this way easier?). Be prepared to cut away the foam and be close to the leak.
5) Is there any harm in leaving the present R-13 insulation stapled into the kneewall bays? It’s already there. I want to do what I can to make that top floor comfortable in the summer, and I’m thinking the insulation might not hurt (the attic will be insulated, but I don’t expect it will be as cool as the living areas in the summer).

Sorry for the long post. Thanks for any suggestions/barbs, etc.

Mark

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Replies

  1. User avatar
    Dana Dorsett | | #1

    Getting rigid foam boards into the kneewall spaces is usually somewhere between being a PITA or downright impossible.

    Unless that 1.5" deep space is vented at both the soffits & ridge, it's usefulness is limited.

    With a simple roofline it may be cheaper ( and definitely easier and better overall) to put used rigid foam (at least 40% of the total R) above the structural roof deck, and some fluff between the rafters. Since you are re-roofing NOW is the opportunity moment!

    Hitting code min on a U-factor basis is pretty easy with 4" of used polyiso foam held down by a nailer deck through screwed to the structural deck, and R20 between 2x6 rafters. (Even R15's between 2x4 rafters might cut it.) The 4" of polyiso would be good for ~R22-R25 on an R-value basis, and about R20 for dew point control, derated for climate. If you go this route use an impermeable self-healing peel'n'stick over the plank roof deck rather than a permeable type, and #30 felt on the nailer deck. A 1x6 facia board at the eaves is usually
    sufficiently cosmetic for 4" foam + nailer deck.

    In the Dayton area Insulation King will almost always have suitable used or factory-blemished goods available. see: http://www.OhioInsulationking.com

    Actively conditioning the kneewall space is neither necessary or particularly desirable.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Mark,
    Dana is correct: It's almost always easier to install rigid foam above the roof sheathing than it is to install rigid foam between the rafters.

    Dana Dorsett seems to think that you plan to install new roofing. I didn't notice where you said that -- but maybe you did.

    If you end up going with the cut-and-cobble approach, you should read the following article (perhaps you already have): Cut-and-Cobble Insulation.

  3. User avatar
    Dana Dorsett | | #3

    I was perhaps inferring too much from ...

    "We're getting rid of the skylights..."

    and

    "Top down: shingles and vapor permeable underlayment, 1x7 decking..."

    Matching existing roofing in the patched-over skylights is nearly impossible, and vapor permeable underlayment isn't standard traditional fare for 1x plank decking, so either...

    ... it's a VERY NEW roof recently installed and he knows what shingles will matched, and he asked what the underlayment was even before buying the house...

    ...or...

    ...replacing the roof is part of the plan. (Cood b rong, offen am...)

    FWIW: Vapor permeable underlayments don't add much resilience in composite shingle roofing stackups, since the shingles themselves are under 0.15 perms (almost a Class-I vapor retarder, a true vapor barrier.) Even with a vapor permeable underlayment the roof deck can't dry toward the exterior through the shingles.

  4. Mark Waldron | | #4

    Yes, I would have ridge vents and intake vents ("Smartvent" under the first course of shingles--there's no soffit of course).

    Yes, we must get new roof shingles, so I can see the attraction of jumping now to above-the-decking insulation. I admit I didn't strongly consider that because it seems so daunting. Doing the job from underneath bay-by-bay is surely a PITA, but it is a bite-size PITA for a DIY project. Maybe I could handle the above-deck insulation approach, I'll read up on the details. My newby questions will be:
    1) Do I have to remove the present shingles? (maybe I could get the roofers to send their guys to do the tearoff, it was part of the bid for the new roof anyway). The roof is 7/12 pitch, getting the foam placed and a new nailer deck installed might be some high adventure for me plus a guy or two.

    2) What would the roofers charge me to put up the polyiso and new nailer deck, plus new fascia board, re-hang the gutters. I'm afraid of the number, but it won't hurt to ask. If they want $7 per linear foot to replace rotted 1X7 planking, I gotta think this price could be a shocker.

    On the plus side: I won't be cutting and cobbling in the attic for a week or so, and I can even leave the present ratty FG in the rafter bays, just repair it a bit.

    Would I need an intact air or vapor barrier on the inside of the attic if I get R22 on the topside (with the foam)?

    Thanks very much.

  5. User avatar
    Dana Dorsett | | #5

    To do the foam-over right with a plank sheathed roof you'll indeed have to strip both the shingles and the felt, get it down to bare wood before applying the peel'n'stick (to guarantee air-tightness).

    I have no way to guess what a local roofing contractor would charge for 4" of foam + nailer in your area. In higher-priced labor markets it may be cheaper to do it with nailbase panels (or not) rather than separate foam + nailer. Get quotes.

    In case you're not familiar with them, here is an example nailbase polyiso panel product:

    https://www.hunterpanels.com/product-documents/hpanels/speciality-products/108-h-shield-nb/file

    At R21 the 4" panel would be good for up to ~R30 of fluff between rafters, in a zone 5A climate

  6. Mark Waldron | | #6

    Dana and Martin,
    Thanks very much for the information. I'm reading the various pieces in HBA and elsewhere about "foam atop the structural deck with a nailing deck over that" approach. I'm sure that any roofer or code official is going to ask for details (fastener schedule, nailing deck sheeting requirements (is 1/2" 4-ply CDX plywood okay?), etc). Can you steer me toward some type of prescriptive guidance or other source/guide that will convince them I'm not just a crazy homeowner who wants to do things differently? Does this method have a catchy name or an organization that serves as a proponent for it? I've seen the articles on the PERSIST and the REMOTE homes, but nothing aside from magazine articles, etc.

    Edited to add: I did find this "Building America" page( https://basc.pnnl.gov/resource-guides/above-deck-rigid-foam-insulation-existing-roofs#quicktabs-guides=0 ) with guidance and refs to applicable codes. It's not quite as explicit as a residential roofer will want before going into uncharted territory (e.g gap the plywood panels on the nailing base? Use plywood clips? Fastener schedule? etc)

    Dana, thanks for the lead to InsulationKing. They've got what I need, and the price is about 1/2 of new. They are a 3 hour drive from me, but if I can't find some used FG-faced polyiso roof decking insulation locally, it might be the best option. I assume 2 layers of 2" polyiso with staggered seams would be preferable to a single 4" thick sheet.

    Mark

  7. User avatar
    Jon R | | #7

    Google below, table 1 for perms of individual asphalt shingles = .9.

    10013915-Deck-Defense-Technical-White-Paper-October-2011.pdf

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Mark,
    Q. "Can you steer me toward some type of prescriptive guidance or other source/guide that will convince them I'm not just a crazy homeowner who wants to do things differently?"

    A. Here is a link to a GBA article on the topic: How to Install Rigid Foam On Top of Roof Sheathing.

    Q. "Does this method have a catchy name or an organization that serves as a proponent for it?"

    A. Not really, but I advise you to look into a product called nailbase. (Nailbase is rigid foam that is adhered in a factory to a facing of OSB. It's like a SIP with OSB on just one side.) If you Google "nailbase installation" or "nailbase installation guide" or "nailbase installation instructions," you'll find instruction booklets published by a variety of nailbase manufacturers.

    Here is an example: ThermalStar Nailbase Installation Instructions.

  9. Mark Waldron | | #9

    Martin,
    Jackpot! Thanks for the links to your previous article on this and to the Nailbase installation guide--that's just what I need. I'm a little surprised that the installation guide doesn't call for screwing the nailbase through to the rafters, that attaching to the original roof deck (planks in my case) is good enough. I wonder how many of these screws will find the gaps between the planks.
    I looked again at the pitch of the roof, it is 9/12. I haven't been on it yet, but I don't think I'll be bringing my roller skates. I'm still concerned about finding a roofer that will do this work at a price that is within my budget. Well, time to start making calls.

    Thanks again!

  10. Mark Waldron | | #10

    Recap: Extreme south edge of Zone 5A, now considering rigid foam on top of structural roof deck:

    If I go with the polyiso/EPS insulation above my structural (plank) wood deck, I have a question about the air barrier that goes above the structural deck and below the rigid foam. Dana wrote:

    To do the foam-over right with a plank sheathed roof you'll indeed have to strip both the shingles and the felt, get it down to bare wood before applying the peel'n'stick (to guarantee air-tightness).

    If I use peel and stick, I assume there will be no drying through that, nor any drying upward (because there will be more peel-and-stick above the top nailer deck). If, however, the lower membrane is an air barrier (e.g. vapor permeable synthetic underlayment) and my rigid insulation is polyiso with figerglass facers (not foil) or EPS, it would seem that there's at least some potential to dry the two wood decks and the foam to the back side.

    Martin, I see that you have recommended (https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing ) either a vapor permeable underlayment OR peel-and-stick for a plank roof deck with insulation on top, so maybe the back-side drying potential isn't important?

    So, I think my questions are:
    1) If I don't incorporate a ventilation channel anywhere above the structural roof deck, am I better off with no water vapor diffusion to either top or bottom, or having some (limited) diffusion through the back side? My gut says it would be better to have some vapor openness to the back side.

    2) If I have no vapor retarder above the lower deck, will I be inviting problems due to water vapor flow going the other way--from my warm conditioned attic into the cold foam?

    Thanks,

    Mark

  11. User avatar GBA Editor
    Martin Holladay | | #11

    Mark,
    With this type of roof assembly, there isn't any justification for worrying much about the vapor permeance of the layers adjacent to the rigid foam layer. The rigid foam is itself either a vapor barrier or close to a vapor barrier -- so adding peel-and-stick adjacent to the rigid foam doesn't really affect the drying ability of the assembly very much.

    In some climates, there is an advantage to providing a ventilation channel between the top layer of rigid foam and the roofing. (Such a ventilation channel can reduce the chance of ice dams.) You can create a 1.5-inch-high ventilation channel above the rigid foam if you want by installing flatways 2x4s, 16 inches or 24 inches on center, but such a ventilation channel is optional.

  12. Mark Waldron | | #12

    An update: I called quite a few roofers, and it appears that the "rigid foam and top nailer deck installed over the existing structural roof deck" is far from the expertise of most residential roofers. After talking to many of these folks, it appears that "unusual" equals "expensive." One extremely knowledgeable roofer with a Cape Cod like mine said he had strongly considered this approach when it was time to put a new roof on his own home, he knew all about it. But, in the end, though he would have liked it, he threw in the towel and insulated from below just due to cost. And it looks like I'll probably do the same thing.
    On the bright side: Most of the ceilings on the first floor are now torn out (sagging, cracked plaster), so cut-and-cobble under the roof deck at the lower portions can be done standing up on a ladder or scaffold/platform rather than creeping along on back/belly atop the ceiling joists. Getting material (including cut-up 1/2" drywall to cover the foam) is relatively easy through the open ceiling joists. And we'll be taking down the drywall on the sloped ceiling portion of the second floor, making it relatively easy to install rigid foam into the stud bays and an inch of continuous foam can be placed under all the rafter bottoms before the drywall goes back up, so approx. 5" total of polyiso (R30, nominally, less when temps get very low and the polyiso performance wanes). I'll also have an approx 1.5" ventilation channel between the insulation and the roof deck (fed by new intake vents below and a ridge vent at top), which is a bit of insurance against ice dams and decking rot.
    It's not the superroof I had wanted, but it's a LOT better than what was there before, it puts the whole attic (including the HVAC equipment) inside the thermal envelope, and it is within my budget and DIY skills (I think!).

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